This invention relates to a method and apparatus for testing a battery, such as an industrial battery, and more particularly, to such a method and apparatus whereby the operating condition and/or characteristics, including possible fault conditions, of the battery is determined.
Often, it is desirable to ascertain whether a battery is defective, or whether it is capable of being charged, or what its charge capacity is, or what its present state-of-charge is. Depending upon these factors, it may be concluded that the battery should be replaced. Also, these factors can be used to establish optimum parameters for charging that battery. For example, in many industrial applications, it is necessary that batteries whose charges have been depleted be quickly re-charged for subsequent re-use. Such industrial-type batteries exhibit high charge capacities, on the order of several hundred ampere-hours (A.H.), and usually are acid-type batteries, such as the conventional lead-acid battery. Desirably, such industrial batteries should be charged by apparatus that is relatively simple to operate.
Some commercially available battery charges employ the so-called constant voltage technique wherein a constant charging voltage is applied across the battery for a pre-set charging duration. However, this technique does not take into account a change in the actual charge capacity of the battery due to repeated charging operations and, thus, becomes less effective as the battery ages. It would be helpful if the battery could be tested to determine the actual charge capacity thereof.
Other commercially available battery charges establish a pre-set charging time which, generally, is unrelated to the actual charge level of the battery. The battery then is charged with a substantially constant charging current which is established by the operator as a function of the rated battery capacity. Since, over a period of time, the actual battery capacity may differ substantially from its rated capacity, the charging current might be too high. Also, if the battery exhibits a relatively low charge level, the charging time duration may be insufficient to charge the battery satisfactorily. It would be advantageous if the charge level, or state-of-charge, of the battery could be determined.
A battery charger has been introduced by Christie Electric Corp., Los Angeles, Calif., wherein the state-of-charge of the battery is measured by, for example, an ammeter, which indicates battery current during interruptions in the charging operation. However, this charger suffers from disadvantages, such as operating over a pre-set charging time duration which may be too short or too long. Also, this charger operates with relatively low capacity batteries.
A battery charger introduced by Westinghouse Davenset Rectifiers of England supplies a charging current to the battery and senses when the battery voltage reaches a predetermined level. This level is assumed to be the gas voltage (V.sub.gas) level which, as is known, is the battery voltage at which the battery exhibits a "gassing" condition wherein the electrolyte of the battery is subjected to electrolysis to emit gas. In the Westinghouse Davenset Rectifiers battery charger, when the battery voltage reaches the gas voltage level V.sub.gas, a pre-set timer is triggered to establish the gas period. At the termination of this pre-set gas period, normal charging is terminated; and the battery then is supplied with an equalizing charge followed by a "hold ready" charge which replaces open circuit losses. While this charger includes various features, such as pre-gas charge protection, avoiding the establishment of the gas period if a fully charged or slightly discharged battery is used, charging nevertheless occurs during a pre-set time interval. There is little, if any, correlation between the actual state-of-charge of the battery and the charging duration. The operating condition and/or characteristics of the battery is not tested. Hence, this charger may, undesirably, either over-charge or under-charge a battery connected thereto.
In the battery charger manufactured by Oldham/Harmer & Simmons, of England, a charging current is supplied to the battery, and this current is measured for a brief period of time when the battery voltage reaches its gas voltage level V.sub.gas. Operation, however, proceeds substantially independently of the actual condition of the battery. The charging current levels are dependent upon the rated battery capacity, which may differ substantially from the actual capacity thereof. Furthermore, the actual battery operating and fault conditions are not sensed automatically or indicated to an operator. Hence, faulty or defective batteries may be charged, with a resultant waste in energy and time. Furthermore, a faulty battery, which may be easily repaired, if not indicated, may be supplied with a charging current that results in permanent damage.
It also has been proposed by the prior art to supply a charging current to a battery, and to interrupt the charging current periodically to measure the battery voltage. This measure of battery voltage is used to indicate whether the battery has reached its gassing level, and the charging current magnitude is reduced when this gassing level is attained. Although the battery voltage is "tested" periodically, such tests are not used to indicate the condition of the battery, nor are the test results used to indicate when the battery has been satisfactorily charged.
The prior art also has proposed a battery test arrangement wherein a current ramp is supplied, resulting in a change in the battery voltage. The voltage-current characteristic for each cell, which is a function of the supplied current and measured voltage, is determined by a computer; and an average voltage-current characteristic is derived from all of the cells. Then, the voltage-current characteristic of each cell is compared to the average voltage-current characteristic, and the battery is rejected if the characteristic of any one cell differs significantly from the average characteristic. However, this arrangement requires direct access to each cell of the battery. In many industrial batteries, such access is difficult, if not impossible. Furthermore, since the battery is "tested" by deriving an average voltage-current characteristic therefrom, a battery which is severely defective will not be detected.
Yet another prior art battery test technique proposes that a voltage-current curve derived from the tested battery be plotted, and that the slope of this curve be compared to an "average" curve which represents average slopes for different battery charge levels. Then, the actual charge level of the battery under test is determined by noting the charge level on this "average" curve corresponding to the measured slope of the test voltage-current curve. This test technique, although helpful in obtaining a measure of the state-of-charge of the battery, nevertheless does not provide indications of various fault conditions which may exist.
Another problem associated with many battery chargers is that the charging current supplied thereby should be matched to the battery which is charged. Typically, it is necessary that the capacity of the battery as well as the number of cells included therein be known in advance, as opposed to being ascertained merely by testing the battery. Furthermore, many of these chargers will attempt to charge a defective battery notwithstanding a serious fault condition which may be present. This can result in damage to the battery as well as produce a hazardous condition.
Therefore, there has been a need for apparatus that is capable of automatically testing the condition of the battery, determining the operating characteristics thereof (e.g. charge capacity, state-of-charge, and the like), and indicating or displaying various fault conditions.